An organic electroluminescent element (may hereinafter be referred to as an organic EL element) is a self light-emitting element, and is thus brighter, better in visibility, and capable of clearer display, than a liquid crystal element. Hence, active researches have been conducted on organic EL elements.
In 1987, C. W. Tang et al. of Eastman Kodak developed a laminated structure element sharing various roles among different materials, thereby imparting practical applicability to organic EL elements using organic materials. They laminated a layer of a fluorophor capable of transporting electrons, and a layer of an organic substance capable of transporting holes, and injecting the charges of electrons and holes into the layer of the fluorophor to perform light emission, thereby obtaining a high luminance of 1,000 cd/m2 or more at a voltage of 10V or less (see Patent Document 1 and Patent Document 2).
Many improvements have been made to date for commercialization of organic EL elements. For example, high efficiency and durability are achieved by an electroluminescent element sharing the various roles among more types of materials, and having a positive electrode, a hole injection layer, a hole transport layer, a light emission layer, an electron transport layer, an electron injection layer, and a negative electrode provided in sequence on a substrate.
For a further increase in the luminous efficiency, it has been attempted to utilize triplet excitons, and the utilization of phosphorescent light emitting compounds has been considered.
Furthermore, elements utilizing light emission by thermally activated delayed fluorescence (TADF) have been developed. An external quantum efficiency of 5.3% has been realized by an element using a thermally activated delayed fluorescence material.
The light emission layer can also be prepared by doping a charge transporting compound, generally called a host material, with a fluorescent compound, a phosphorescent light emitting compound, or a material radiating delayed fluorescence. The selection of the organic material in the organic EL element greatly affects the characteristics of the element, such as efficiency and durability.
With the organic EL element, the charges injected from both electrodes recombine in the light emission layer to obtain light emission, and how efficiently the charges of the holes and the electrons are passed on to the light emission layer is of importance. Hole injecting properties are enhanced, and electron mobility is increased to increase the probability of holes and electrons recombining and, moreover, excitons generated within the light emission layer are confined, whereby a high luminous efficiency can be obtained. Thus, the role of the electron transport material is so important that there has been a desire for an electron transport material having high electron injection properties, allowing marked electron mobility, possessing high hole blocking properties, and having high durability to holes.
In connection with the life of the element, heat resistance and amorphism of the material are also important. A material with low thermal resistance is thermally decomposed even at a low temperature by heat produced during element driving, and the material deteriorates. In a material with low amorphism, crystallization of a thin film occurs even in a short time, and the element deteriorates. Thus, high resistance to heat and satisfactory amorphism are required of the material to be used.
A representative light emitting material, tris (8-hydroxyquinoline)aluminum (will hereinafter be abbreviated as Alq3) , is generally used as an electron transport material as well. However, the work function of Alq3 is 5.8 eV, and cannot be said to have hole blocking performance.
As a measure for preventing some of the holes from passing through the light emission layer and increasing the probability of charge recombination in the light emission layer, there is a method of inserting a hole blocking layer. As hole blocking materials, triazole derivatives (see Patent Document 3), bathocuproine (will hereinafter be abbreviated as BCP), and aluminum-mixed ligand complexes {for example, aluminum (III) bis (2-methyl-8-quinolinato)-4-phenylphenolate (will hereinafter be abbreviated as BAlq)} have so far been proposed.
As an electron transport material excellent in hole blocking properties, 3-(4-biphenylyl)-4-phenyl-5-(4-t-butylphenyl)-1,2,4-triazole (will hereinafter be abbreviated as TAZ) has been proposed (see Patent Document 3).
TAZ has a great work function of 6.6 eV, indicating a high hole blocking ability. Thus, when used as an electron-transporting hole blocking layer, to be laminated on the negative electrode side, for a fluorescent light emitting layer or a phosphorescent light emitting layer which are prepared, for example, by vacuum deposition or coating, TAZ contributes to an increase in the efficiency of the organic EL element.
Low electron transporting properties, however, are a major problem with TAZ, and there is need to combine TAZ with an electron transport material having higher electron transporting properties, thereby preparing an organic EL element.
BCP also has a work function as great as 6.7 eV, and has a high hole blocking ability. However, its glass transition point (Tg) is so low (83° C.) that its thin film is scarcely stable, and BCP cannot be said to function fully as a hole blocking layer.
In short, all the above materials are either lacking in film stability, or insufficient in the function of blocking holes. In order to improve the element characteristics of the organic EL element, there has been a desire for an organic compound excellent in electron injection/transport performance and hole blocking capability and highly stable in a thin film state.
As a compound improved in such defects, a compound having a benzopyridoindole ring structure has been proposed (see Patent Document 4).
However, an element using the compound of Patent Document 4 for an electron injection layer and/or an electron transport layer has been improved in luminous efficiency, but the improvement has been still insufficient. Thus, an even lower driving voltage, an even higher luminous efficiency and, in particular, an even higher current efficiency, have been desired.